1. Field of the Invention
The present invention relates to a vacuum unit, and more specifically to a vacuum unit having monitoring and control features associated therewith.
2. Description of the Related Art
Heretofore, pressure switches for negative pressure and/or positive pressure have widely been employed in a pneumatic apparatus or the like.
A description will now be made of one example illustrative of a pressure switch for negative pressure, which is actuated to convert a pressure value into a pressure signal or an electric signal. Such a pressure switch is employed in a negative pressure (vacuum) system including pressurized air, a fluid passage and a pneumatic apparatus or the like. That is, the pressure switch is used to confirm whether a subject or object (workpiece) to be fed has been attracted by or released from a suction pad or cup.
The pressure switch is provided with a pressure switch signal generating circuit comprising a carrier diffusion type semiconductor pressure sensor, a comparator, a variable resistor, an output circuit (for generating a pulse signal), etc. The pressure switch signal generating circuit generates an output signal S.sub.1 corresponding to a differential pressure developed due to a change in pressure (see M in FIG. 1) at the time of suction and release (i.e., vacuum break) of the workpiece. Incidentally, the differential pressure is provided to avoid undesired chattering action.
Now, a threshold value PH (corresponding to the leading edge of the output signal S.sub.1) of the differential pressure shown in FIG. 1 is compared with a reference value set by the variable resistor by using the comparator supplied with the output signal S.sub.1 from the carrier diffusion type semiconductor pressure sensor. The output signal S.sub.1 is supplied to a sequence controller operable with a computer for FA (Factory Automation) and the like, for example, so as to be used for various control driving means or the like. Further, the output signal S.sub.1 is used as a module for a CIM and enables a control operation including a determining or judging function.
In the above prior art, however, the pressure switch is not normally provided with a vacuum displaying means. Therefore, a process for attracting the workpiece or releasing the same is repeatedly performed when the threshold value PH or the like is determined. Then, the variable resistor or the like is used to set and adjust each of corresponding pressure values so as to determine the threshold value PH. Thus, when a number of pressure switches are disposed in the pneumatic apparatus or the like, it is necessary for the switches to drive the pneumatic apparatus or the like and carry out the setting and adjustment of the corresponding pressure values, both of which require a relatively large amount of effort. Further, there is developed an error between each of the corresponding pressure value and a preset value, thus creating difficulty in quantitatively setting the pressure values. Furthermore, the pressure values are set and adjusted in analog form, thereby causing inconvenience, in that it is difficult to accurately set the pressure values.
Further, even if a normal pressure displaying means is used, a display confirming operation itself is rendered difficult because the representation of a set pressure value is made in plane form upon its visual recognition.